"Since you last visited" cue + PWA install (add to home screen)

Two calm returning-reader features.

Since-last-visit (Highlights companion, not a nav lane — per Codex):
* queries.feed gains a `since` filter; GET /api/since?ts= returns the count +
  a few accepted/non-dup/visible articles discovered since the reader's last
  visit (boundary-respecting; invalid/future ts → 0, no error).
* Home stores last_seen in localStorage (reads prev, then stamps now); on
  Highlights, a gentle "Since you were last here, N new calm reads came in"
  note with a "See what's new" reveal of a compact inline section. Dismissible.
  No badges, no unread counts, no "missed" language.

PWA:
* Real PNG icons (192/512 + full-bleed maskable) rasterized from favicon.svg;
  manifest fixed (azure theme to match the brand, PNG icons); apple-touch-icon.
* Minimal service worker: precache the app shell, always-fresh API + /a/ pages.
* Gentle, dismissible install banner (beforeinstallprompt → Install; iOS → the
  Share → Add to Home Screen hint). Never nags.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_admin.py

@@ -265,3 +265,19 @@ def test_follows_and_following_feed(tmp_path, monkeypatch):
     assert TestClient(app).get("/api/feed?following=true").json()["count"] == 0
     assert TestClient(app).get("/api/follows").status_code == 401
     assert tc.post("/api/follows", json={"kind": "source", "value": "999"}).status_code == 404
+
+
+def test_since_endpoint(tmp_path, monkeypatch):
+    import os, sqlite3
+    app, api = _make(tmp_path, monkeypatch, admin_email="boss@x.com")
+    c = sqlite3.connect(os.environ["GOODNEWS_DB"])
+    for aid, when in [(2, "2020-01-01 00:00:00"), (3, "2030-01-01 00:00:00")]:
+        c.execute("INSERT INTO articles (id,source_id,canonical_url,title,url_hash,discovered_at) VALUES (?,1,?,?,?,?)",
+                  (aid, f"http://s/{aid}", f"t{aid}", f"h{aid}", when))
+        c.execute("INSERT INTO article_scores (article_id,accepted) VALUES (?,1)", (aid,))
+    c.commit(); c.close()
+    tc = TestClient(app)
+    r = tc.get("/api/since?ts=2027-01-01T00:00:00Z").json()
+    assert r["count"] == 1 and [i["id"] for i in r["items"]] == [3]   # only the post-2027 article
+    assert tc.get("/api/since?ts=2099-01-01T00:00:00Z").json()["count"] == 0  # nothing newer
+    assert tc.get("/api/since?ts=not-a-date").json()["count"] == 0            # invalid ts → quiet 0